I got this error when adding a path to the "Statically cache specific pages" box. I added a path, and hit save and received the error. Also, all of the paths I had in that box are now gone, so I have to go through and re-add them.

UPDATE: I re-added all the paths that I don't want cached and it doesn't seem like I'm getting the error anymore. Strange, but I'll update if it reappears.